In today's world, radio spectrum is a scarce resource. The Federal Communications Commission's (FCC) chart of radio frequency allocation shows that almost all available spectrum has already been heavily licensed. But actual measurements taken by the FCC's Spectrum Policy Task Force have shown that most of current radio spectrum is underutilized and lies vacant most of the time. Cognitive Radios (CR) have been proposed as a revolutionary communication paradigm to address this problem of apparent spectrum scarcity. CRs operate by opportunistically using underutilized spectrum while not degrading the operational quality of the licensed users. In the CR context, the licensed user of a band is called the Primary User (PU) while the cognitive radio operating on the same band is called the Secondary User (SU).